1. Field of the Invention
The present invention relates to a technique for determining a hibernation period of a communication apparatus.
2. Description of the Related Art
Wireless communication methods that perform independent distributed communication, such as WiMedia, have been proposed in recent years. WiMedia was standardized as Standard ECMA-368 (High Rate Ultra Wideband PHY and MAC standard) by the European Computer Manufacturers Association (ECMA). FIG. 7 shows an exemplary arrangement of beacon groups consisting of a WiMedia device and other, neighboring WiMedia devices. The WiMedia devices transmit beacons to and from one another within the beacon group and reserve their respective communication bands. Each device transmits a beacon in each beacon slot of a beacon band (beacon period) located in the first time period of a superframe having a predetermine time length as shown in FIG. 8. BG1 shown in FIG. 8 is a beacon group formed by device A1. BG2, BG3, BG4, . . . , BG8 are beacon groups formed by devices A2, A3, A4, . . . , A8 respectively. Beacons are transmitted by the devices with a time lag between the devices, because of variations in clock precision among the devices. Therefore, all devices are in accordance with the device whose superframe start time (Beacon Period Start Time, hereinafter referred to as BPST) is the latest to be synchronized with the other devices. Each device detects a Beacon Period Occupied Information Element (BPOIE) in a beacon transmitted by another device, and can identify a device in the beacon group of the device, based on the BPOIE thereof. Each device synchronizes with devices within a beacon group constituted by the device, and devices within a beacon group constituted by each device in the beacon group constituted by the device, namely an extended beacon group (EGB). For example, device A1 synchronizes with any of the beacon groups (BG2, BG4, BG6, and BG8) constituted by any of the devices (A2, A4, A6, and A8) within the beacon group to which device A1 belongs. The start time of a superframe of each device can be obtained by calculating the BPST from the sequential number of a beacon transmitted by the device (beacon slot number), and the time of reception of the beacon.
Portable apparatuses such as digital cameras and PDAs are becoming equipped with wireless devices in recent times. Such portable apparatuses in general are battery-powered and have stringent power-consumption requirements. Therefore, hibernation mode for reducing power consumption is an essential function for wireless communication.
Hibernation mode for reducing power consumption is provided in WiMedia as well. However, resynchronization after waking from hibernation mode is difficult to accomplish in independent distributed communication such as WiMedia. The reason is that, when a WiMedia device enters hibernation mode, the BPST of the device in hibernation mode gradually falls out of synchronization with the BPST of the other devices operating in the beacon group to which that device belongs, due to differences in clock precision among the devices.
Therefore, WiMedia makes it mandatory to perform the following processing concerning wakeup from hibernation mode. A device in hibernation mode receives a beacon from another device in the frame immediately before waking up. The device must set its BPST in accordance with the latest BPST among the beacons received, and transmit a beacon in accordance with the set BPST to wake up. On the other hand, a device notified by a device of its intention to enter hibernation mode reserves a beacon slot for the device in hibernation mode, so that the device can restart sending a beacon in the beacon slot when waking up from hibernation mode.
However, multiple beacon groups can fall out of synchronization, because a beacon cannot be received from the device that entered hibernation mode in the same beacon group. For example, when device A1 in FIG. 7 enters hibernation mode, the beacon group BG1 dissolves. As a result, the beacon group BG2 constituted by the device A2 and the beacon group BG4 constituted by the device A4 no longer have to synchronize with each other and will start independent synchronization in the beacon group.
FIG. 9 shows points at which devices in beacon groups transmit beacons while the device A1 is in hibernation mode. The devices A2, A4, A6, and A8 that belonged to the beacon group BG1 before the device A1 entered hibernation mode have reserved a beacon slot for the device A1 in hibernation mode. Because the beacon slot reserved by the device A1 for the device A6 is emptied, the device A8 shifts beacon slots. The BPSTs of the devices (A2, A8, and A3) in the beacon group BG2 and the devices (A4, A5, and A6) in the beacon group BG4 fall out of synchronization because the beacon groups start synchronizing independently of one another.
Consider a case where the device A1 wakes up from hibernation mode in the state where multiple beacon groups start independent synchronization as described above. The device A1 will receive a beacon from another device (A2, A4, A6, A8) in a beacon band of the superframe immediately before waking up. However, according to WiMedia, when the time offset (the gap) between the BPSTs of two beacon groups that can receive each other's beacons exceeds a predetermined time (24 microseconds), the beacon groups should be merged into one. Therefore, if the time offset between BPSTs of the two beacon groups BG2 and BG4 has exceeded a permissible value (24 microseconds) for beacon synchronization, merging is performed when the device A1 wakes up and the beacon group BG1 is re-formed. Thus, the beacon groups cannot immediately synchronize with each other.
A method for merging the beacon group BG1 with the beacon groups of the devices that belonged to the beacon group BG1 will be described with reference to FIGS. 12A through 12C. The device A1 transmits a beacon in synchronization with one of the beacon groups BG2 and BG4 when the device A1 wakes up. It is assumed herein that the device A1 transmits a beacon in synchronization with the beacon group BG2.
In WiMedia, when the beacon groups whose beacon periods overlap each other are to be merged, the beacon group that has the later BPST shifts the beacon to the slot succeeding the beacon slot of the beacon group having the earlier BPST. The devices A4 and A6 in FIG. 12A, which have received a beacon of the device A1, wait for a random number of superframes and then transmit beacons in the beacon slots succeeding the beacon slot of the device A8, thereby avoiding collision between the beacons during merging (FIG. 12B).
The device A5 does not transmit a beacon for a while after the device A4 received the beacon of the device A1 and therefore can communicate only with the device A7 during the time period. When the device A4 starts transmitting a beacon in synchronization with the device A1, the device A5 becomes unable to transmit a beacon for a time period equivalent to a random number of superframes. The device A5 will subsequently transmit a beacon in synchronization with the device A4 in the beacon slot succeeding the beacon slot of the device A6 (FIG. 12C). The devices A4 and A6 can shift the beacon slots for which other devices have not reserved bands while the device A5 is merging (FIG. 12C).
If other synchronization relationships exist between beacon groups, the merging of one beacon group can have a ripple effect on other beacon groups. Consequently, merging between beacon groups can significantly restrict communication and reduce communication efficiency.